This invention relates to a magnetron and more particularly to a structure for fixing a gasket which shields leakage of the high frequency output.
Generally, the magnetron is incorporated in electronic ovens or defrosters and widely used for heating or defrosting foodstuffs since it efficiently generates high frequency energy.
FIG. 1 shows, in schematic sectional view, a typical, conventional magnetron. In the figure, reference numeral 1 denotes an anode cylinder made of, for instance, oxygen-free copper with a plurality of radial vanes 2 fixed on the inner wall thereof. The anode cylinder 1 and the vanes 2 constitute an anode electrode of the magnetron. Reference numerals 4 and 5 denote conical pole pieces fixed at the upper and lower ends of the anode cylinder 1. A sealing member 6 in the form of a cup is fixed by, for instance, soldering to the upper surface of the magnetic pole piece 4 and an insulating member 7 is provided above the sealing member. On the central axis of the anode electrode 3, there is provided a thermionic emission type cathode electrode 8 of, for instance, coiled thorium tungsten in a space surrounded by the free end portions of the plural vanes 2, defining a so-called interaction space 9 between the cathode electrode 8 and the tips of the vanes 2. Reference numerals 10 and 11 denote end shields provided at both ends of the cathode electrode 8 respectively, for preventing electrons from deviating axially.
Reference numerals 12a and 12b denote side supports, 12c a center support. These supports pass through the insulating member 7 with one end supported thereby. Secured to the other end of the supports is the cathode electrode 8. Heating current is supplied to the cathode electrode 8 through these supports. Reference numerals 13a through 13c denote sealing members through which the supports 12a through 12c are air-tightly fixed to the insulating member 7, and 14 an insulating spacer. The insulating spacer 14, through which the supports 12a to 12c pass, serves to prevent vibratory motion of the supports and to correct the location of the cathode electrode 8. Reference numeral 15 denotes a sealing metal member fixed to the magnetic pole piece 5, and 16 an insulating dome which extends from the sealing metal member 15. An antenna 17 in the form of a metal rod connected, at one end, to the vane 2 is inserted in the dome 16. The antenna 17 serves to pick up high frequency energy and deliver it to the outside. Reference numerals 18 and 19 respectively denote permanent magnets, 20 and 21 yokes to introduce the magnetic field produced by the permanent magnets 18 and 19 into the interaction space 9, and 22 and 23 external lead wires connected to the supports 12a and 12c. To the tip of the lead wire 22 is connected an L-C filter circuit comprising a high-voltage capacitor 27 and a coil 26 which is wound on a ferrite core 24 through, for instance, an adhesive 25 of silicon rubber group. Thus, the input terminal of the magnetron and the L-C filter circuit are enclosed with a filter case 28 and shielded against high frequency.
Reference numeral 29 denotes a metal gasket inserted in a annular space defined by the sealing metal member 15, the yoke 21 and the end surface of the magnetic pole piece 5. The gasket 29 generally takes the form of a ring which is formed by weaving a mesh of stainless steel wire, brass wire or the like with excellent elasticity and electrical conductivity. The metal gasket 29 is urged against the input end portion of a wave guide (not shown) when the magnetron is incorporated into an electronic oven so as to prevent high frequency wave from leaking from the antenna 17 to the outside of the wave guide.
In operation, when a predetermined operating current is supplied between the lead wires 22 and 23 to heat the cathode electrode 8 and a predetermined voltage is supplied to the anode 3, the magnetron starts to oscillate at high frequency and the high frequency energy is radiated into, for instance, an electronic oven through the antenna 17 and the wave guide.
However, in the magnetron of such a conventional structure, the metal gasket 29 inserted in the annular space defined by the sealing metal member 15, the yoke 21 and the pole piece 5 is usually held in the annular space by elasticity of its own and frictional force until the input end portion of the wave guide (partly illustrated at phantom lines in FIG. 2) is connected to the yoke 21 as shown in FIG. 2. However, since the metal gasket has an extremely low dimensional precision due to the fact that it is formed by weaving metal wires and since the sealing metal member 15 and the yoke 21 which define the annular space often deform during pressing work, it is extremely difficult to stably hold the gasket 29 in the annular space. Accordingly, the gasket tends to fall off when the magnetron is being transported or mounted. When the magnetron is deprived of the gasket, there occur leakage and radiation of high frequency energy through the annular space, which is hazardous to human body, a cause for radio interference or sparks in the output unit of the magnetron which may break down the magnetron, so that the reliability of the magnetron is drastically lowered.